1. Field of the Invention
The present invention relates to a laser beam irradiation method and a laser irradiation apparatus for using the method (apparatus including a laser and an optical system for guiding laser beam emitted from the laser to an object to be irradiated). In addition, the present invention relates to a method of manufacturing a semiconductor device, which includes a laser beam irradiation step. Note that a semiconductor device described here includes an electro-optical device such as a liquid crystal display device or a light-emitting device and an electronic device that includes the electro-optical device as a part.
2. Description of the Related Art
In recent years, a wide study has been made on a technique in which laser annealing is performed for a semiconductor film formed on an insulating substrate made of glass or the like, to crystallize the film, to improve its crystallinity so that a crystalline semiconductor film is obtained, or to activate an impurity element. Note that a crystalline semiconductor film in this specification indicates a semiconductor film in which a crystallized region is present, and also includes a semiconductor film that is crystallized as a whole.
A method of forming pulse laser beam from an excimer laser or the like by an optical system such that it becomes a square spot of several cm or a linear shape of 100 mm or more in length on an irradiation surface, and scanning the laser beam (or relatively shifting an irradiation position of the laser beam with respect to the irradiation surface) to conduct annealing is superior in mass productivity and is excellent in technology. The “linear shape” described here means not a “line” in the strict sense but a rectangle (or a prolate ellipsoid shape) having a high aspect ratio. For example, it indicates a shape having an aspect ratio of 10 or more (preferably, 100 to 10000). Note that the linear shape is used to obtain an energy density required for sufficiently annealing an object to be irradiated. Thus, if sufficient annealing is conducted for the object to be irradiated, it may be a rectangular shape.
FIGS. 27A and 27B show an example of a configuration of an optical system for forming laser beam in a linear shape on an irradiation surface. This configuration is extremely general. All optical systems described above are based on the configuration shown in FIGS. 27A and 27B. According to the configuration, a cross sectional shape of laser beam is converted into a linear shape, and simultaneously an energy density distribution of laser beam on the irradiation surface is homogenized. In general, an optical system for homogenizing the energy density distribution of laser beam is called a beam homogenizer.
Laser beam emitted from a laser 101 is divided in a direction perpendicular to an optical axis the laser beam by a cylindrical lens array 103. The direction is called a first direction in this specification. It is assumed that, when a mirror is inserted in a course of an optical system, the first direction is changed in accordance with a direction of light bent by the mirror. In this configuration, the cylindrical lens array is divided into seven parts. Then, the laser beams are superposed on an irradiation surface 109 by a cylindrical lens 104, thereby homogenizing an energy density distribution of the linear laser beam in the longitudinal direction, and the length of the longitudinal direction is determined.
Next, the configuration shown in the side view of FIG. 27B will be described. Laser beam emitted from a laser 101 is divided in a direction perpendicular to an optical axis thereof and the first direction by cylindrical lens arrays 102a and 102b. The direction is called a second direction in this specification. It is assumed that, when a mirror is inserted in a course of an optical system, the second direction is changed in accordance with a direction of light bent by the mirror. In this configuration, the cylindrical lens arrays 102a and 102b each are divided into four parts. The divided laser beams are temporarily synthesized by a cylindrical lens 104. After that, the laser beams are reflected by a mirror 107 and then condensed by a doublet cylindrical lens 108 so that they become again single laser beam on the irradiation surface 109. The doublet cylindrical lens 108 is a lens composed of two cylindrical lenses. Thus, an energy density distribution of the linear laser beam in a width direction is homogenized, thereby homogenizing an energy density distribution of the linear laser beam in the longitudinal direction, and the length of the width direction is determined.
For example, an excimer laser in which a size in a laser window is 10 mm×30 mm (which each are a half-width in beam profile) is used as the laser 101 and laser beam is produced by the optical system having the configuration shown in FIGS. 27A and 27B. Then, linear laser beam which has a uniform energy density distribution and a size of 125 mm×0.4 mm can be obtained on the irradiation surface 109.
At this time, when, for example, quartz is used for all base materials of the optical system, high transmittance is obtained. Note that coating is preferably conducted for the optical system such that transmittance of 99% or more is obtained at a frequency of the used excimer laser.
Then, the linear laser beam formed by the above configuration is irradiated with an overlap state while being gradually shifted in a width direction thereof. Thus, when laser annealing is performed for the entire surface of an amorphous semiconductor film, the amorphous semiconductor film can be crystallized, crystallinity can be improved to obtain a crystalline semiconductor film, or an impurity element can be activated.
In addition, an area of a substrate used for manufacturing a semiconductor device is being increased more and more. This is because high throughput and a low cost can be realized in the case where a plurality of semiconductor devices such as liquid crystal display device panels are manufactured from a single large area substrate as compared with, for example, the case where a semiconductor device such as liquid crystal display device panel is manufactured from a single substrate. At the present time, for example, a substrate of 600 mm×720 mm, a circular substrate of 12 inches (about 300 mm in diameter), etc. are used as the large area substrate. Further, it is expected that a substrate in which a length of one side exceeds 1000 mm will be also used in future.
However, in the optical system forming a linear beam that is longer than the prior art, for example, 300 mm, in the length direction, the optical path length thereof is as long as 5,000 mm. It is extremely difficult and requires a large footprint to perform optical adjustment of the optical system with such a long optical path length, thereby causing a problem in that the apparatus becomes larger.
Now, a part of the optical system is changed, for example, a lens with a short focal distance is substituted for the lens that is used to change the distance between the lenses, and the optical path length of the optical system forming the linear beam of 300 mm in the length direction, is set to, for example, 2,400 mm. However, in the linear beam formed by the optical system, a convergent position deviates at both ends of the irradiation surface in the length direction.
Here, the description will be made of the cause that the convergent position deviates at both ends of the irradiation surface in the length direction by making the optical path length short. The optical path length of light incident obliquely upon the lens is longer than that of light incident perpendicularly thereupon. Also, as the incident angle of the obliquely incident light is larger, the optical path difference from the perpendicularly incident light is larger. The difference of the optical path length depending on the incident position or incident angle becomes deviation of the convergent position. Accordingly, in the irradiation surface, an image thereon is fuzzier in its part closer to the end of the laser beam, that is, curvature of field develops in which an image is formed on a curved surface as shown in FIG. 28. If such a linear beam is used to perform annealing on an irradiation subject, annealing can not be uniformly performed.
In addition, under the present circumstances where substrate areas are increasing greatly, there is an urgent need to form the linear beam of approximately 1000 mm in the length direction. For example, in case of performing annealing on a large area substrate, if the linear beam of 300 mm in the length direction is used, it is impossible to perform annealing on the entire surface of the large area substrate with only one scanning by irradiating the linear beam to the large area substrate while moving the beam in one direction relative to the substrate, and it is required to move the beam in at least two directions or to irradiate the beam plural times, thereby reducing the throughput. As a result, depending on a scanning method of the laser beam, there is formed a region where annealing is performed plural times or a region where no annealing is performed, so that there is a strong possibility that the annealing is not uniformly performed. When the optical system is therefore devised which forms the linear beam having such a length in the length direction that makes it possible to perform annealing on the entire surface by moving the beam in only one direction along the large area substrate, for example, a length of 1000 mm, supposing the optical path length is in a range where it can be used in practice, the convergent position deviates at both ends of the irradiation surface in the length direction.
The cause that the convergent position deviates at both ends of the irradiation surface in the length direction by forming the laser beam which is long in the length direction is similar to that by making the optical path length short. It is because the difference of the optical path length depending on the incident position or incident angle becomes deviation of the convergent position and the curvature of field develops. Even if the linear beam is made longer in the length direction in order to perform annealing uniformly, as long as such a linear beam with the convergent position deviating at both ends in the length direction is used, annealing can not be uniformly performed.